1) Field of the lnvention
The present invention relates to a conductive foam rubber roller used in an image formation apparatus such as a copying machine, a laser printer, a facsimile or the like, in which an electrostatic latent image is electrostatically developed with a developer.
2) Description of the Related Art
Generally, in an image formation apparatus such as an electrophotographic recording apparatus, the following processes are typically carried out:
a) a uniform distribution of electrical charges is produced on a surface of an electrostatic latent image carrying body;
b) an electrostatic latent image is formed on a charged area of the body surface by an optical writing means such as a laser beam scanner, an LED (light emitting diode) array, a liquid crystal shutter array or the like;
c) the latent image is developed as a visible image with a developer or toner, which is electrically charged to be electrostatically adhered to the latent image zone;
d) the developed and charged toner image is electrostatically transferred from the body to a recording medium such as a cut sheet paper; and
e) the transferred toner image is fixed and recorded on the cut sheet paper by a toner image fixing means such as a heat roller.
Typically, the electrostatic latent image carrying body may be an electrophotographic photoreceptor, usually formed as a drum, called a photosensitive drum, having a cylindrical conductive substrate and a photoconductive insulating film bonded to a cylindrical surface thereof. In the charging process, an electric discharger such as a corona discharger is widely used to produce the charged area on the photosensitive drum, and this type of discharger is also used in the transferring process in which the developed and charged toner image is electrostatically transferred from the drum to the paper. The electric discharger has an inherent defect in that ozone is produced during the energizing thereof. Not only is ozone injurious to the health, but also it causes a premature deterioration of the drum and other parts of the electrophotographic recording apparatus.
For this reason, in place of the electric discharger, a conductive foam rubber roller is recently used as an electric charging roller in the charging and transferring processes. The electric charging roller is in contact with the photosensitive drum, and is connected to a suitable electric source so that the drum or the paper is charged with a desired polarity. Of course, during the charging of the drum or the paper by the electric charging roller, no ozone is produced.
Note, the electric charging roller must be resiliently pressed against the drum to obtain a given contact or nip width therebetween, before the drum or the paper can be sufficiently charged by the electric charging roller.
As one type of developer, a non-magnetic type one-component developer is known, which is composed of only a toner component (colored fine synthetic resin particles). In a developing device using this type developer, a conductive foam rubber roller may be used as a developing roller. The conductive developing roller is rotated within a body of the developer held by a vessel, and a portion of the developing roller exposed therefrom and is in contact with the photosensitive drum. In the developing process, the toner particles are frictionally entrained by the developing roller to be brought to the surface of the photosensitive drum for development of the latent image. In this case, in the developing device using the one-component developer, it is necessary to bring the toner to the drum at a uniform thickness before an even development of the latent image can be obtained. Namely, a uniform layer of the toner must be formed around the developing roller. To this end, the developing device is provided with a blade member engaged with the surface of the developing roller, to uniformly regulate a thickness of the toner layer formed therearound. The blade member also serves to electrically charge the toner particles by a triboelectrification therebetween, and a material of the blade member is selected such that the toner is charged with a desired polarity. Alternatively, the blade member is formed of a conductive material such as metal, and is connected to a suitable electric source to electrically charge the toner particles by a charge-injection effect. During the developing process, the developing roller is supplied with a developing bias voltage from a suitable electric source so that the charged toner particles are electrostatically attracted to only the latent image zone.
Note, a toner density of the developed toner image depends upon a contact or nip width between the developing roller and the photosensitive drum. Namely, in general, the larger the nip width between the roller and the drum, the higher the toner density of the developed image. Accordingly, the developing roller must be pressed against the drum at a suitable liner pressure before it is possible to obtain a proper density of the developed toner image.
The developing device may also include another conductive foam rubber roller used as a toner-removing roller. The toner-removing roller is resiliently pressed against the developing roller, and is rotated in the same direction as the developing roller so that the surfaces of the rollers rub against each other in reverse directions at a contact zone therebetween, whereby residual toner particles not used for the development of the latent image are mechanically removed from the developing roller. The toner-developing roller is supplied with a voltage from a suitable electric source so that a penetration of the charged toner particles into the toner-removing foam rubber roller is electrostatically prevented.
The conductive foam rubber roller used as the charging roller and the developing roller must have a suitable softness so that an operating life of the photosensitive drum can be extended as long as possible, because the harder the conductive foam rubber roller which is resiliently pressed against the drum, the greater a wear of the photoconductive insulating film of the drum. Also, the conductive foam rubber roller used as the toner-removing roller should have a suitable softness, because, if the toner-removing roller is harder than the developing roller, the latter is prematurely worn by the toner-removing roller which is resiliently pressed thereagainst. Furthermore, the conductive foam rubber roller must have pore openings a diameter of which is at most twice an average diameter of the toner particles, so that a penetration of the toner particles thereinto can be effectively prevented, because the roller is hardened by this penetration. Since typically the toner particles have an average diameter of about 10 .mu.m, the pore openings of the roller should be given a diameter of less than 20 .mu.m, preferably 10 .mu.m.
Conventionally, the conductive foam rubber roller having pore openings, the diameter of which is less than 20 .mu.m, may be produced by the following processes:
a) a suitable resin material, e.g., polyurethane, silicone, acrylonitrile-butadiene or the like, containing a conductive substance such as carbon or a fine metal powder, and a water-soluble foam-providing substance such as polyvinyl alcohol or methyl cellulose is extruded as a long tubular product;
b) the extruded tubular product is immersed in a body of water held by a container, and thus the water-soluble foam-providing substance dissolves in the body of water so that a foam structure is given the tubular product;
c) the tubular foam product is cut into tubular roller elements having a predetermined length; and
d) the tubular roller element is mounted and fixed on a metal shaft member by using a suitable adhesive such as a thermosetting adhesive, to thereby produce a conductive foam rubber roller.
This conductive foam rubber roller must be further treated before it can be used as a charging roller, developing roller, or toner-removing roller as mentioned above, because the tubular roller element is covered at an outer surface thereof by a solid skin layer having a thickness of about 1 to 5 .mu.m and the content of the conductive substance is very small in comparison with that of the conductive substance in the internal foam structure of the tubular element. Namely, the conductive foam rubber roller is finished by removing the solid skin layer from the tubular roller element thereof.
Nevertheless, many of the finished conductive foam rubber rollers cannot have a desired electric characteristic when an electric energy is applied to the tubular roller element through the metal shaft member, due to an existence of an inner solid skin layer formed at an inner wall surface of the central bore of the tubular roller element. In particular, an electric resistivity of the inner solid skin layer is considerably higher than that of the foam structure of the tubular roller element because because a content of the conductive substance therein also is very small, and a thickness of the inner solid skin layer is variable along a longitudinal axis of the metal shaft member. Thus, an electric potential of the tubular roller element, which is obtained by the application of the electric energy to the tubular roller element through the metal shaft member, is also variable along the longitudinal axis of the metal shaft member. Accordingly, for example when this defective conductive foam rubber roller is used as a developing roller, it is impossible to obtain an even development of the latent image.